Designing, Building, and Installing a 3D Background in an Established Aquarium

My 150 gallon aquarium started out with humble origins as bare bottom tank for my painted turtle. It's still bare bottom and still has the turtle, but now we've added fish and some decorations. Since it's in our living room we really want to make it look nicer inside. My thought was to add a 3D background.

It seems to me that the 3D background needs to be made out of drylok and not concrete since drylok doesn't affect pH. The other issue is that I'm not sure how to secure it in place. It seems everyone glues it in with silicone, but that isn't going to work in an established tank. My thought was to create this whole thing with foam and then scrap the foam out once the drylok has cured. To ensure that the background remains sturdy sans foam I'd then need to add concrete and then cover that concrete with drylok. At this point the background will be heavy and it shouldn't float or want to move. A single piece would be easier in every way, except installation. Installing it would be essentially impossible unless this was in multiple parts, since removing the brace does not make me comfortable.

Anyone have experience making and adding a 3D background to an established tank

You can always move the fish and turtle to some cheap plastic totes for a few days while you remodel the tank...

Lots of options on the construction of a background, there is no single right or wrong way to make it or any single right or wrong materials to use... A lot of your decisions will be based on what you are comfortable using...

Cement can be hardened so that it has minimal effect on PH, or it can be sealed with epoxy or a clear coating paint...

You can always move the fish and turtle to some cheap plastic totes for a few days while you remodel the tank...

Lots of options on the construction of a background, there is no single right or wrong way to make it or any single right or wrong materials to use... A lot of your decisions will be based on what you are comfortable using...

Cement can be hardened so that it has minimal effect on PH, or it can be sealed with epoxy or a clear coating paint...